Catalytic cracking is a major conversion process in the petroleum industry. Within a catalytic cracking reactor a refinery fuel oil feedstock and an active catalytic agent are reacted under heat to convert the fuel to produce lighter, more valuable products. Broadly speaking the heavy fuel feedstock contains mixtures of aromatic, naphthene and paraffin molecules with a common feed being a virgin gas oil, which is the part of the crude oil boiling between 600.degree. F. and 1050.degree. F. Cracking of the feedstock generally occurs in the reactor at temperatures within the range of about 890.degree. F. to 1025.degree. F. The spent catalyst is processed to regenerate it and to be recycled in the catalytic cracking process stream. The effluent from the catalytic cracking reactor is sent into a fractionator in which the gaseous, light oil and heavy oil products are separated. The cat cracker bottoms are the heaviest and highest boiling fractions and contain some catalysts fines which must be removed or reduced in level to allow for the cat cracker bottoms to be blended into a fuel oil or used as needle coke or carbon black feedstock.
A number of catalysts having a high activity which are satisfactory for commercial cracking may be used. The catalysts may be natural or synthetic, with the main types being amorphous and zeolitic. A common catalyst is a crystalline zeolite, or molecular sieve catalyst, which is generally impregnated on an amorphous clay or a silica-alumina base. Another common catalyst is an amorphous or noncrystalline type comprising alumina and silica, or alternatively silica and magnesia.
The cat cracker bottoms which contain the catalyst fines are commonly routed to a settling tank wherein the solid particles (catalyst fines) are allowed to settle by gravity with the upper layer of substantially fines-free cat cracker bottoms being decanted off for product use. Generally, the incoming stream of cat cracker bottoms contain an ash level of about 0.1 to 0.5% by weight and the level of fines must be reduced by at least 75% or greater to an ash level of less than 0.05% by weight for subsequent product use of the cat cracker bottoms. After settling, the resultant sludge containing the catalyst fines and the cat cracker bottoms hydrocarbons will generally have an ash content (fines level) on the order of 10 to 40% by weight and a cat cracker bottoms hydrocarbon level on the order of 60 to 90% by weight. The resultant sludge not only contains a large amount of useful cat cracker bottoms hydrocarbons, but presents difficult and expenseive disposal problems.
Another method of reducing the level of fines in the cat cracker bottoms is by dielectrophoretic separation, the resultant cat cracker bottoms sludge having an ash level on the order of 2 to 5% by weight.
Past practice involved transporting the sludge to landfarms, landfills or other disposal treatments. The landfarm process consists of controlled application and cultivation of waste on soil, on a properly engineered site, in order to use microbes naturally present in the soil to decompose the organic fraction of the waste. These past practices have been extremely time consuming and expensive, with the need being identified for a more efficient and environmentally safe process to be developed.
U.S. Pat. No. 4,123,357 discloses a process whereby oil, water, and the solids materials of the sludge are stirred at high temperatures of at least 95.degree. F., followed by settling, decanting, settling and again decanting the upper layers which are substantially solid free. U.S. Pat. No. 4,206,001 discloses circulating a solvent with the sludge in the rundown tank, followed by decanting the solvent, then washing the settled solids with water, followed again by decanting the aqueous solution and hydrocarbons. While these procedures are effective in reducing the level of cat cracker bottoms on the solids, an alternative process is desirable which can more efficiently remove the cat cracker bottoms or other hydrocarbons from the catalyst fines and which is less time consuming.
Therefore, it is a feature of the present invention to provide a process which efficiently removes hydrocarbons from cat cracker bottoms sludge.
It is another feature of this invention to treat the catalyst fines of cat cracker bottoms sludge to provide vide an environmentally safe treated fine.
It is still a further feature of this invention to recover and isolate the cat cracker bottoms hydrocarbons from the cat cracker bottoms sludge.